The major goal of this research proposal is to examine the influence of age and gender on single muscle fiber contractile properties before and after 12-wks of progressive resistance training (PRT). Our work during the past three years (NIA AG15486) has provided insight to the cellular adaptations in 70-yr. old men and women before and after resistance training. However, data describing the contractile properties of single muscle fibers in men and women over 80 yr. is unknown. We have preliminary data showing that adults over 80 yr. have a continued reduction in contractile speed and that the fast-twitch (type II) fibers are selectively altered. This is an important finding that may help explain the accelerated muscle strength losses (approximately 30 percent per decade) observed after the age of 70 yr. In addition, single fiber contractile properties before and after PRT have not been characterized in young adults. Therefore, the intent of this research will be to extend our single muscle fiber studies to include young (20-30 yr) and very old (>80 yr) men and women. Before and after a 12-wk PRT program, type I (slow) and II (fast) muscle fibers will be analyzed for size (microm), peak force (Po), shortening velocity (Vo), force-velocity relationship, and power. We will test the hypotheses that: 1) There will be a progressive decline by age (young to very old) in the individual slow and fast-twitch single muscle fiber contractile function before and after PRT. This progressive decline in contractile function will be more pronounced in the fast-twitch muscle fibers (hypotheses 1-3), and 2) Gender differences in single fiber contractile function will be present in the older men and women, but not the younger men and women (hypothesis #4). Upon completion of this proposed research, along with our previous work on moderately old adults, we will have single muscle cell data before and after PRT in young (<30 yr), moderately old (approximately 70 yr), and very old (>80 yr) men and women. Together, these studies will provide a detailed understanding of sarcopenia at the cell level (i.e. fiber specific - slow vs. fast fibers) which are not detectable using whole muscle measurements. With this information we will be able to determine, for the first time, the interaction of aging, resistance training, and gender on single muscle fiber contractile function.